The invention pertains to prevention of power interruptions, and more particularly a flywheel energy storage device, for preventing power loss to a DC bus, having high reliability, long life, and is more tolerant to extreme operating environments and load demands than conventional electrochemical batteries. The flywheel uses a brushless permanent magnet motor/generator powered by a closed loop operated synchronous inverter to charge, and an unregulated rectifier is used for discharging to the DC bus. In certain applications, the invention can provide a lower cost, simpler and more reliable flywheel energy device than competing conventional regulated flywheel systems.
Power reliability, in the area of telecommunications in particular, is of utmost importance. The electric utility grid in the United States is currently 99.9% reliable. This means that the utility power is down an average of 8 hours per year. With the advent of the Internet, e-commerce, and widespread computer connectivity, this loss of power for telecommunications is currently deemed unacceptable. To prevent loss of telecommunications during interruption of the utility grid power, uninterruptible power supplies (UPS) are installed at distributed locations for back up. The UPS that are currently available commercially and accepted as the standard, all rely on lead acid batteries for storage of the back up energy. Valve-regulated lead acid batteries, particularly used in outside plant locations, have terrible reliability as well as longevity properties due to the lack environmental control. Furthermore, they require periodic maintenance and replacement.
Flywheel energy storage systems are now becoming recognized as having the potential to offer more reliable reserve power than conventional electrochemical batteries. However, current designs are usually more expensive and not suitable for all types of applications. In the area of telecommunications, there exists a need for a very low cost device that can be deployed widespread and can prevent power interruptions both in locations where auxiliary generating means are not available and for ride-thru until an auxiliary generator has time to start. Such a device could power outside plant network electronics with the standard direct current telecommunications voltages.
In most designs of flywheel energy systems, power from the flywheel generator is regulated to advantageously increase the amount of energy that can be extracted from the rotating flywheel. The voltage of a motor/generator, whether separate units or combined, is directly proportional to the rotational speed, so as the speed of the flywheel slows during a discharge, the voltage falls and ultimately decreases to below a useable level. To reduce the cost, weight and size of the flywheel, some regulation method is employed so that the flywheel generator can supply constant voltage, even as the speed of the flywheel is slowing. Two methods to accomplish this regulation are through electronic switching or through the use of a variably energizable field coil such as in an alternator type motor/generator.
For permanent magnet type motor/generators, electronic switching can be done in a motor drive inverter such that it is made to operate in 4th quadrant mode during discharging. The motor drive thereby actively boosts and regulates the voltage from the generator during discharging so as to maintain a constant output voltage for a wide speed range, extracting most of the energy from the flywheel. Unfortunately, in some applications, especially high power ones, the cost of the motor drive to handle the high power discharge can be high. This is despite the fact that charging in most applications can be satisfactorily accomplished at a slower rate with a low power motor drive. The output switching regulation also incurs switching losses.
A second method for electronic output voltage regulation is to use a separate wide range DC-DC converter. As the flywheel speed falls, the converter maintains a constant voltage to a DC bus. This method can also extract almost all of the energy from the flywheel; however, it can suffer equally from high power electronics costs as the 4th quadrant operated method. Likewise, the efficiency can be comparable.
To reduce the cost of providing output voltage regulation, an alternator type motor/generator or one with a variably excitable field coil can be employed. Since the output voltage from a generator is a function of both the speed and the field flux density, the output voltage can be maintained constant as the flywheel slows by increasing power to the field coil. A simple and more reliable method for extracting up to 90% of the flywheel""s energy is achieved. However, to control the output, a field controller is used which continuously consumes power and adds some cost along with the motor/generator structure. This method can also provide another potential path for system failure.
The invention provides a high reliability flywheel energy storage device for preventing power interruptions to a DC bus. The device has longer life and is more tolerant of extreme operating environments and high power load demands than conventional electrochemical batteries. In certain applications, this invention can provide a lower cost, simpler and more reliable flywheel energy device than competing conventional regulated flywheel systems. This can be true particularly in flywheel system applications where the cost of the flywheel is a smaller portion of the complete system, where output power regulation electronics costs are significant, or where the size and weight of the flywheel system is not of significant importance.
The device uses a flywheel with an attached or integral permanent magnet brushless motor/generator. The motor/generator is electrically energized by a closed operated synchronous inverter that supplies synchronous charging power to the armature coils to accelerate the flywheel to normal operating speed. Because flywheel can be satisfactorily charged or accelerated at a low rate, the input power to the flywheel during charging can be substantially lower than the output power from the flywheel during discharging. For this reason, and also because the synchronous inverter is not required to switch or boost the unregulated output voltage, the synchronous inverter can be made small to save substantial costs, if desired. During an interruption of primary utility power, the flywheel motor/generator supplies unregulated voltage power to the DC bus through a rectifier that connects the armature coils to the DC bus. As the flywheel slows, the voltage at the DC bus falls, and eventually falls to below a useable level. Use of such a flywheel system without regulation to extract more energy from the flywheel by allowing extraction to low speeds is contrary to the conventions well known in the art. However, the amount of the usable energy delivered to the DC bus by the device can be increased the by the combination a low voltage drop in the armature coils of the motor/generator and by building the flywheel with sufficient inertia to reduce the rate of decay of the back emf of the motor/generator to the rate desired for the application.
Another aspect of the invention is the fact, generally overlooked in this industry, that most, if not all, equipment that currently is backed up by electrochemical batteries, is made to operate on a varying range of voltage. This is true because the voltage of batteries varies significantly with their state of charge. For lead acid batteries, the voltage swing from charging to discharge can be 25% or more. This allows some range of voltage and an upper speed range in which the flywheel can discharge useable energy to the load without voltage regulation. Hence, a constant output voltage from the flywheel system for these applications is not required. Adding to these facts are that, although the voltage of a motor/generator is linearly proportional to its operating speed, the energy stored is proportional to the square of its operating speed. A small allowable operating voltage range can allow a proportionally larger extraction of the flywheel""s energy, because much of the energy stored in the spinning flywheel is extracted at the top end of the flywheel""s speed range.
The voltage drop in the armature coils is the result of the discharge current multiplied by the resistance or impedance of those coils, which subtracts directly from the fully charged motor/generator back emf reducing extractable energy at the instant discharging starts. In one a preferred embodiment, to insure a low voltage drop in the motor/generator, the armature coils are designed with an electrical resistance per phase, R, in ohms, equal to:
Rxe2x89xa60.15 (V/I)
where V is the normal DC bus voltage, in volts, during normal operation of the primary utility power, and I is the normal current, in amps, through the DC bus during normal operation of the primary utility power.
In a second embodiment, the rate of slowing of the flywheel is reduced by designing the flywheel for capability of supplying useable power to the DC bus for a time, T, in seconds by building the flywheel with a rotational moment of inertia, IN, in kg m2, and normal operating speed, w, in radians per second, such that:
INxe2x89xa73.6 V I T/w2.
Alternatively, the flywheel device delivers useable power to the DC bus only in the speed range from the normal operating speed to some lower speed that is greater than 60% of the normal operating speed.
To attain these conditions, a much larger flywheel is used for a given energy delivery than those well known in the art, and hence is contrary to prior teachings. The cost, weight and size of the flywheel can be substantially increased even by a factor of two or more in some cases. However, it has been found that of the numerous types of flywheel systems and their numerous applications, that the invention can, in some applications, offer substantial reduction in complete system cost, standby power consumption, and an increase in reliability and energy delivery efficiency. This is particularly true in applications where the flywheel cost is a smaller portion of the system cost, the electronics regulation costs are substantial, and where the flywheel size and weight are not of considerable importance. One potential application is in low cost distributed telecommunications network power systems while others exist in high power ride-thru for generator sets.